Viral induction, colicine production, prokaryotic cell division and oncogenesis are induced by the same class of agents, i.e., radiation, mitomycin, deprivation of thymine, chemicals that affect DNA synthesis and repair, etc. and may operate through common molecular mechanisms. The writer proposed a mechanism of prophage induction which may have applicability to the other systems mentioned above. Lambda repressor is assumed to have affinity not only for lambda operators, but also for single-strand lesions in the host DNA. Induction would occur when the capacity of the cell to repair the lesions is exceeded. Under these conditions, repressors and recA protein would bind to the single strand gaps and proteolytic cleavage of the repressors would take place at that site. We have established that lambda repressor has affinity for ssDNA gaps in vitro and the non-inducible ind- repressor has less affinity. We propose: 1) to introduce gapped plasmid DNA into lysogens by transformation and quantitate the induction caused; 2) to examine the effect of gapppd DNA on proteolytic cleavage of repressor; 3) to determine if inds mutant repressor has higher affinity for gapped DNA; 4) to prepare monoclonal antibodies against a fragment of repressor; and 5) to isolate mutants deficient in the proteolytic cleavage of repressor to find out whether the induction can occur by the presence of ssDNA gaps only.